Syntheses and crystal structures of four 4-(4-nitrophenyl)piperazinium salts with hydrogen succinate, 4-aminobenzoate, 2-(4-chlorophenyl)acetate and 2,3,4,5,6-pentafluorobenzoate anions

The syntheses and low-temperature crystal structures of four organic salts of 4-(4-nitrophenyl)piperazine are presented.


Chemical context
4-Nitrophenylpiperazinium chloride monohydrate has been used as an intermediate in the synthesis of anticancer drugs, transcriptase inhibitors and antifungal reagents (Berkheij et al., 2005;Chaudhary et al., 2006;Kharb et al., 2012;Upadhayaya et al., 2004). It is also an important reagent for potassium channel openers, which show significant biomolecular current-voltage rectification characteristics (Lu, 2007). The design, synthesis and biological profiling of arylpiperazine-based scaffolds for the management of androgensensitive prostatic disorders was described by Gupta et al. (2016). 4-Nitrophenylpiperazine was the starting material in the synthesis and biological evaluation of new piperazinecontaining hydrazone derivatives (Kaya et al., 2016). A review on the piperazine skeleton in the structural modification of natural products was recently published by Zhang et al. (2021).

Figure 3
The molecular structure (50% displacement ellipsoids) of III. Hydrogen atoms are shown as arbitrary circles. The dashed line indicates a hydrogen bond.

Figure 1
The molecular structure (50% displacement ellipsoids) of I. Hydrogen atoms are shown as arbitrary circles. The dashed line indicates a hydrogen bond.

Figure 4
The molecular structure (50% displacement ellipsoids) of IV. Hydrogen atoms are shown as arbitrary circles. The dashed line indicates a hydrogen bond.
Throughout all four structures, individual bond lengths and angles take on normal values except for an elongated O-H bond [1.17 (2) Å ] in I, which will be described in more detail in the next section (Supramolecular features).

Supramolecular features
Hydrogen bonding plays a significant role in the packing of all four salts (see Tables 1-4). In each structure, the asymmetric units were chosen to give the shortest hydrogen bonds between the cationic NH 2 group and the anionic carboxylate groups. In I, II, and IV, these hydrogen bonds to the anion are equatorial relative to the piperazine ring, while that in III is axial. Nevertheless, in each structure, the NH 2 + group acts as a hydrogen-bond donor through both its hydrogen atoms. In I, III, and IV this is to a second anion, whereas in II it is to the included water molecule. Throughout the four structures, all conventional N-HÁ Á ÁO and all but one O-HÁ Á ÁO (in I, vide infra) hydrogen bonds take on normal distances and angles (Tables 1-4).
Structure II also includes N-HÁ Á ÁO hydrogen bonds from the 4-amino group of its anion to the nitro oxygen atoms of its cation ( Table 2). The cation-anion interactions, along with the presence of the water molecule, which acts as an O-HÁ Á ÁO hydrogen-bond donor to join a pair of translation-related (1 + x, y, z) anions and as an acceptor for an N-HÁ Á ÁO hydrogen bond, generates a double-layer network lying parallel to (011) (Fig. 7). Of the four structures, II has the most complex hydrogen-bonding interactions.
The primary supramolecular interaction in III joins two pairs of inversion-related ammonium cations and carboxylate anions, forming an R 4 4 (12) ring motif (Table 3, Fig. 8). Structure III also includes the onlyinteractions of the four structures, which occurs between inversion-related (Àx, 1 À y, Àz) nitrophenyl rings, giving an interplanar spacing of 3.3352 (15) Å , though the offset ('1.92 Å ) is large, leading to a centroid-centroid distance of 3.8495 (15) Å (Fig. 8, dashed line). Supramolecular interactions within IV are the simplest of the four structures: N-HÁ Á ÁO hydrogen bonds connect cations and anions into continuous chains that extend parallel to its a-axis. These interactions are quantified in Table 4

Refinement
Crystal data, data collection, and structure refinement details are given in Table 5. All hydrogen atoms were found in difference-Fourier maps, but those bound to carbon were subsequently included in the refinement using riding models, with constrained distances set to 0.95 Å (Csp 2 -H) and 0.99 Å (R 2 CH 2 ), using U iso (H) values constrained to 1.2U eq of the attached carbon atom. All N-H and O-H hydrogen atoms were refined freely (both coordinates and U iso ). For all structures, data collection: APEX3 (Bruker, 2016); cell refinement: APEX3 (Bruker, 2016); data reduction: APEX3 (Bruker, 2016); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2019/2 (Sheldrick, 2015b); molecular graphics: XP in SHELXTL (Sheldrick, 2008); software used to prepare material for publication: publCIF (Westrip, 2010).  Special details Experimental. The crystal was mounted using polyisobutene oil on the tip of a fine glass fibre, which was fastened in a copper mounting pin with electrical solder. It was placed directly into the cold gas stream of a liquid-nitrogen based cryostat (Hope, 1994;Parkin & Hope, 1998). Diffraction data were collected with the crystal at 90K, which is standard practice in this laboratory for the majority of flash-cooled crystals. Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement progress was checked using Platon (Spek, 2020) and by an R-tensor (Parkin, 2000 (12) 0.34399 (7) 0.0124 (2) Atomic displacement parameters (Å 2 )

Special details
Experimental. The crystal was mounted using polyisobutene oil on the tip of a fine glass fibre, which was fastened in a copper mounting pin with electrical solder. It was placed directly into the cold gas stream of a liquid-nitrogen based cryostat (Hope, 1994;Parkin & Hope, 1998). Diffraction data were collected with the crystal at 90K, which is standard practice in this laboratory for the majority of flash-cooled crystals. Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement progress was checked using Platon (Spek, 2020) and by an R-tensor (Parkin, 2000

4-(4-Nitrophenyl)piperazinium 2-(4-chlorophenyl)acetate (III)
Crystal data Special details Experimental. The crystal was mounted using polyisobutene oil on the tip of a fine glass fibre, which was fastened in a copper mounting pin with electrical solder. It was placed directly into the cold gas stream of a liquid-nitrogen based cryostat (Hope, 1994;Parkin & Hope, 1998). Diffraction data were collected with the crystal at 90K, which is standard practice in this laboratory for the majority of flash-cooled crystals. Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement progress was checked using Platon (Spek, 2020) and by an R-tensor (Parkin, 2000 (2) Special details Experimental. The crystal was mounted using polyisobutene oil on the tip of a fine glass fibre, which was fastened in a copper mounting pin with electrical solder. It was placed directly into the cold gas stream of a liquid-nitrogen based cryostat (Hope, 1994;Parkin & Hope, 1998). Diffraction data were collected with the crystal at 90K, which is standard practice in this laboratory for the majority of flash-cooled crystals. Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement progress was checked using Platon (Spek, 2020) and by an R-tensor (Parkin, 2000